1. Field of the Invention
The present invention relates to tool steels and more particularly to a process for producing tool steels using chemically prepared, substantially pure vanadium trioxide, V.sub.2 O.sub.3, as a vanadium additive. In a more specific aspect, the invention relates to the production of tool steels having an intermediate or high carbon content, i.e., above about 0.35 weight percent.
Tool steels are generally produced with a high carbon content, e.g. as high as 5.0 weight percent in some instances. They also contain alloy elements such as vanadium, tungsten, chromium, molybdenum, manganese, aluminum, silicon, cobalt, and nickel. Typically, the vanadium content of tool steels ranges from about 0.4 to 5 weight percent.
Throughout the specification and claims, reference will be made to the term "chemically prepared V.sub.2 O.sub.3 ". This vanadium trioxide is prepared according to the teachings of D. M. Hausen et. al, in U.S. Pat. No. 3,410,652 issued on Nov. 12, 1968, the disclosure of which is incorporated herein by reference. As described in that patent, V.sub.2 O.sub.3 is produced by a process wherein a charge of ammonium metavanadate (AMV) is thermally decomposed in a reaction zone at elevated temperatures (e.g. 580.degree. C. to 950.degree. C.) in the absence of oxygen. This reaction produces gaseous by-products which provide a reducing atmosphere. The V.sub.2 O.sub.3 is formed by maintaining the charge in contact with this reducing atmosphere for a sufficient time to complete the reduction. The final product is substantially pure V.sub.2 O.sub.3 containing less than 0.01 percent vanadium nitride. V.sub.2 O.sub.3 is the only phase detectable by X-ray diffraction.
2. Description of the Prior Art
It is common practice to alloy steel with vanadium by adding ferrovanadium or vanadium carbide (VC-V.sub.2 C) to the molten steel. The ferrovanadium is commonly produced by the aluminothermal reduction of vanadium pentoxide (V.sub.2 O.sub.5) or by the reduction of a vanadium-bearing slag or vanadium-bearing residue, for example. Vanadium carbide is usually made in several stages, i.e., vanadium pentoxide or ammonium vanadate is reduced to vanadium trioxide, V.sub.2 O.sub.3, which in turn is reduced in the presence of carbon to vanadium carbide under reduced pressure at elevated temperatures, (e.g. about 1400.degree. C.). A commercial VC-V.sub.2 C additive is produced by Union Carbide Corporation under the trade name "Caravan".
Vanadium additions have also been made by adding vanadium oxide, e.g. V.sub.2 O.sub.5 or V.sub.2 O.sub.3, to the molten steel along with a reducing agent. For example, U.S. Pat. No. 4,361,442 issued to G. M. Faulring et. al on Nov. 30, 1982, discloses a process for adding vanadium to steel wherein an addition agent consisting of an agglomerated mixture of finely divided V.sub.2 O.sub.5 and a calcium-bearing material, e.g. calcium-silicon alloy, is added to the molten steel preferably in the form of a molded briquet.
U.S. Pat. No. 4,396,425 issued to G. M. Faulring et al. on Aug. 2, 1983, discloses a similar process for adding vanadium to steel wherein the addition agent is an agglomerated mixture of finely divided V.sub.2 O.sub.3 and calcium-bearing material.
U.S. Pat. No. 3,591,367 issued to F. H. Perfect on July 6, 1971, discloses a vanadium addition agent for use in producing ferrous alloys, which comprises a mixture of vanadium oxide, e.g., V.sub.2 O.sub.5 or V.sub.2 O.sub.3, an inorganic reducing agent such as Al or Si, and lime. The purpose of the lime is to flux inclusions, e.g. oxides of the reducing agent, and to produce low melting oxidic inclusions that are easily removed from the molten steel.
Vanadium addition agents of the prior art, while highly effective in many respects, suffer from a common limitation in that they often contain residual metals which may be harmful or detrimental to the steel. Even in those cases where the addition agent employs essentially pure vanadium oxide e.g. V.sub.2 O.sub.3, the reducing agent usually contains a significant amount of metallic impurities. This problem is particularly troublesome in tool steels, which require relatively high levels of vanadium addition.